Cosmetic cleanup in explain.c, to reduce diff vs. upstream.
Cosmetic cleanup in explain.c, to reduce diff vs. upstream.

* Remove unnecessary #includes, and extern declaration for
  Test_print_direct_dispatch_info

* Remove unneeded debugging code to print target list (you get that in
  a nicer format with EXPLAIN VERBOSE now)

* Remove duplicated assignment of 'useprefix' variable in show_sort_keys

* Whitespace fixes.

ashwinstar/gpdb,jmcatamney/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,edespino/gpdb,lisakowen/gpdb,ashwinstar/gpdb,Chibin/gpdb,edespino/gpdb,jmcatamney/gpdb,50wu/gpdb,edespino/gpdb,Chibin/gpdb,edespino/gpdb,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,edespino/gpdb,Chibin/gpdb,lisakowen/gpdb,50wu/gpdb,Chibin/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,edespino/gpdb,edespino/gpdb,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,edespino/gpdb,greenplum-db/gpdb,Chibin/gpdb,adam8157/gpdb,edespino/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,edespino/gpdb,50wu/gpdb,Chibin/gpdb,adam8157/gpdb,jmcatamney/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,Chibin/gpdb,xinzweb/gpdb,Chibin/gpdb,Chibin/gpdb,jmcatamney/gpdb,Chibin/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,jmcatamney/gpdb